What is the strongest natural fiber?

Spider silk, which is tougher than steel by weight - meaning it absorbs more energy before breaking than almost anything natural or manufactured, including Kevlar. Toughness is the relevant measure rather than tensile strength: silk is not the hardest to snap, it is the hardest to break because it stretches enormously while resisting.

A single spider makes several kinds for different jobs. Dragline silk for the web frame and safety line is the strong one; capture spiral silk is stretchy and coated in glue; and there are separate silks for wrapping prey and for egg cases. The protein is stored as a liquid and turns solid as it is drawn through a duct that changes acidity and pulls the molecules into alignment - the spinning is the hard part to copy, not the chemistry.

Which is why nobody farms it. Spiders are territorial and cannibalistic and cannot be kept at density, so the one large demonstration piece - a golden shawl made in Madagascar in the 2000s - took over a million spiders collected each morning, milked, released, and eight years of work. The effort has gone into producing the protein in bacteria, yeast, goats and genetically modified silkworms instead, with silkworms coming closest because they already have the machinery to spin a fibre.